One time programmable (OTP) electrically programmed read only memory (EPROM) can be an effective, low cost technology for providing non-volatile memory in a variety of CMOS or PMOS processes. A basic OTP-EPROM cell can be a simple enhancement-mode p-channel MOS field effect transistor (MOSFET) built in an n-well like any other p-channel MOSFET except that there is no connection to the poly gate. The OTP-EPROM cell can use a thin gate oxide (e.g., 75 Å to 200 Å range), no special masks or process steps, and is available in the base line process with no cost added to the basic CMOS or PMOS wafer price. However, special test requirements, multiple passes through a multi-probe, and the use of ultra violet (UV) light to erase the OTP-EPROM cell can increase the cost of a probed OTP-EPROM ready for assembly.
An OTP-EPROM cell can be designed using standard semi-conductor design rules. The minimum channel length allowed of an OTP-EPROM cell can be the same or slightly larger than the channel length of a p-channel MOS transistor. For example, if the minimum channel length of a p-channel MOS transistor is 0.4 μm, then the channel length of an OTP-EPROM cell can be 0.5 μm.
An OTP-EPROM cell can be programmed using a nominal voltage supply (e.g., 7.0 V) and can be expected to retain data through a 10 year operating life time at temperatures up to about 125° C. The power supply can be applied during the programming pulses. The read voltages should be kept below about 2.0 V to avoid any possibility of disturbance or degradation of data retention characteristics.
OTP-EPROM cells manufactured using CMOS processes with standard buried channel PMOS have a fairly high Ioff current (e.g., I virgin current) compared to the Ion current (e.g., I programmed current). The high Ioff current of the OTP EPROM cell can reduce the operating margin of OTP-EPROM cell and can reduce its lifetime.
The Ion/Ioff ratio of an OTP-EPROM cell formed using buried channel PMOS technology can be improved by increasing the channel length of the OTP-EPROM. Increasing the channel length, however, usually results in the formation of an OTP-EPROM cell with a larger area. An OTP-EPROM cell with a larger area is undesirable as it prevents the formation of OTP-EPROMs with greater bit density. The Ion/Ioff ratio of the OTP-EPROM cell can also be improved by increasing the programmed state current of the OTP-EPROM cell or decreasing the leakage of virgin state current of the OTP-EPROM cell. Increasing the programmed state current is generally undesirable because a corresponding increased programming voltage and increased programming time are required. It is thus desirable to reduce the leakage of virgin state current in order to reduce the Ion/Ioff ratio and improve the operating margin of the OTP-EPROM.